1. Field of the Invention
This invention relates to enhanced, mobility buried channel transistor structures. Accordingly, it is a general object of this invention to provide new and improved structures of such character.
2. General Background
Enhanced mobility devices using Al.sub.x Ga.sub.1-x As/GaAs materials systems have been reported upon by others and their potential use as high-speed active devices have been recognized. For example, Mimura et al., in an article entitled "A New Field-Effect Transistor with Selectively Doped GaAs/n-Al.sub.x Ga.sub.1-x As Heterojunctions", Japanese Journal of Applied Physics, Vol. 19, No. 5, May, 1980, pp. 1225-1227, describe studies of field-effect control of high mobility electrons in MBE (molecular beam epitaxy)grown selectively doped GaAs/n-Al.sub.x Ga.sub.1-x As heterojunctions. A fabrication of a field-effect transistor, termed a high electron mobility transistor (HEMT), with extremely high speed microwave capabilities is reported therein. As reported, in heterostructure junctions, including alternate layers of GaAs and Al.sub.x Ga.sub.1-x As, with Al.sub.x Ga.sub.1-x As layers being selectively doped with silicon, due to higher electron affinity of GaAs, free electrons in the Al.sub.x Ga.sub.1-x As layers are transferred to the non-doped GaAs layers where they form a quasi two-dimensional Fermi gas. The mobility enhancement behavior is attributed to the spatial separation between the electrons and their parent donor impurities.
Hiyamizu et al. published a paper entitled "MBE-Grown GaAs/N-AlGaAs Heterojunctions and Their Application to High Electron Mobility Transistors" in Proceedings of the 13th Conference on Solid State Devices, Tokyo, 1981, Japanese Journal of Applied Physics, Vol. 21 (1982) Supplement 21-1, pp. 161-168. They noted that selectively doped GaAs/N-AlGaAs heterostructures grown by MBE have attracted much interest because of their potential for application to high speed devices since the quasi two-dimensional electron gas (2DEG) accumulating at the heterojunction interface shows extremely high mobility, especially at low temperatures. In their paper, they describe heterostructures which show high 2DEG mobilities for such heterostructure materials, and fabrication of high speed devices.
The following is a brief review of enhanced mobility transistor operation:
A basic enhanced mobility transistor structure includes a semi-insulating substrate formed of group III-V material; a preferred form, as discussed hereinafter, would include gallium arsenide. As indicated above, gallium arsenide is a semi-insulator.
In order to provide a clear description of a semiinsulator, it is noted that semiconductors are a broad class of materials which have a moderate bandgap, inbetween that of conductors and insulators. Through doping, or through the addition or introduction of impurities to various semiconductors, material can be produced that is either semi-insulating, heavily conducting, or moderately conducting. Thus, the electrical conduction properties of a semiconductor can be tailored to desired specifications by the addition of an appropriate dopant or dopants. In the instant situation, the semi-insulating gallium arsenide substrate, referred to hereinabove, is one which has no intentional conducting dopant added thereto. The semi-insulating gallium arsenide substrate behaves as if it were an insulator, for all practical purposes, and as a means for isolating various electrical circuits from one another.
The basic enhanced mobility transistor structure includes a semi-insulating gallium arsenide substrate with a thin layer of undoped gallium arsenide epitaxially deposited thereon. It further includes a heavily doped n type Al.sub.x Ga.sub.1-x As layer which acts as a carrier source and a thin (50-100 angstroms, for example) layer of undoped Al.sub.x Ga.sub.1-x As between the heavily n doped carrier layer and the GaAs epitaxial layer immediately below.
An enhanced mobility transistor device operates on a principle of modulation doping. Modulation doping occurs when a heavily doped wide bandgap material is placed adjacent to a non-intentionally doped narrow bandgap substance. The operating principle of an enhanced mobility device is dependent upon a conducting sheet of charge in the form of a quasi two-dimensional electron gas (2DEG) which is located at the interface between the narrow bandgap material and the wide bandgap material.
Carriers diffuse from the heavily doped Al.sub.x Ga.sub.1-x As layer into the undoped GaAs layer forming a sheet of negative charge (2DEG) at the Al.sub.x Ga.sub.1-x As/GaAs interface. The 2DEG then acts as a channel, or conducting path, from source to drain (as in a depletion mode field effect transistor) under control of the gate depletion region. Reverse biasing the gate with a sufficiently negative potential interrupts the sheet of negative charge resulting in channel pinchoff, turning off the transistor. The presence of the quasi 2-dimensional electron gas in the nominally undoped GaAs results in reduced ionized impurity scattering and the phenomenon of "enhanced" mobility, i.e., a much large .mu..sub.e than for GaAs doped to an equivalent carrier concentration.
The magnitude of mobility enhancement is reduced by long range coulomb interaction with the ionized impurities and interface scattering. Coulomb interaction is the force exerted between charged bodies. Interface scattering is the mechanism by which carriers moving through a semiconductor are interfered by the discontinuity that the interface represents.